Air-steam Mixture Research Articles

Numerical study of condensing a small concentration of vapour inside a vertical tube

The purpose of this study is to analyse the combined heat and mass transfer of liquid film condensation from a small steam–air mixtures flowing downward along a vertical tube. Both liquid and gas stream are approached by two coupled laminar boundary layer. An implicit finite difference method is employed to solve the coupled governing equations for liquid film and gas flow together with the interfacial matching conditions. The effects of a wide range of changes of three independent variables (inlet pressure, inlet Reynolds number and wall temperature) on the concentration at exit tube, local Nusselt and Sherwood numbers, film thickness, accumulated condensate rate and temperature are carefully examined. The numerical results indicate that in the case of condensing a small concentration of vapours from a mixture, the resistance to heat and mass transfer by non-condensable gas becomes very intense. The comparisons of average Nusselt number and local condensate heat transfer coefficient with the literature results are in good agreement.

Effect of steam content in the air–steam flow on biomass entrained flow gasification

In this work, several experimental schedules have been carried out in order to study the effect of the addition of steam to air as gasifying agent in biomass entrained flow gasification, a promising technology due to its commercial availability, high efficiency and high potential for the production of biofuels and chemicals from biomass. Dealcoholised marc of grape (an abundant residue in the southern regions of Europe with interesting physical and thermochemical properties) has been used as fuel in all the tests. Firstly, the steam content of the gasifying mixture has been varied from 0% (air gasification) to 100% (steam gasification) by keeping the molar fuel/gasifying agent ratio (F) constant, in order to determine the role of air and steam in the gasification process. On the other hand, the effect of the steam/biomass (S/B) ratio has been determined both for steam and air–steam gasification. Lastly, the effect of the operating temperature has been compared both for air and air–steam gasification. The performance of the gasification process has been evaluated through some characterisation parameters, such as the producer gas composition and heating value, the gas yield, the cold gas efficiency, and the product gas ratios and production. The results obtained show that there is an optimal range in the steam content of the gasifying agent (found for air–steam mixtures containing 40–70% mol steam) for which a trade-off between gas quality, gas production, and cold gas efficiency is reached. In general, the addition of steam proved positive for the process performance, not only because of the lower dilution of gas in N2 from air, but also because of the promotion of the steam reforming and the WGS reactions. An increase in the operating temperature has different effects depending on the gasifying agent used. Thus, a higher temperature increases the CO and H2 content in the product gas for air gasification, whereas air–steam gasification leads to a boost in the H2 production at higher temperatures, as well as an increase in the CH4 content.

Investigations in the field of flotation with a steam-air mixture

The influence of the induction and relaxation time of the deformed adsorption layer of the bubble on the stability of the wetting film is investigated theoretically and experimentally. It is shown that, during flotation by a mixture of dry air with the saturated water vapor (aerosol), the adherence selectivity increases due to stabilization of the wetting film. A method for computing the vapor flow rate during flotation with aerosol that does not require empirical information is developed.

Study of the oxidative desulphurization process of coal with different metamorphism degrees

AbstractThe oxidative desulphurization process of coal with different metamorphism degrees treated by an air-steam mixture has been studied. It has been shown that the pyrite present in black coal and anthracite is oxidized with the sulphur dioxide formation, and the process chemical mechanism does not depend on the quality of organic matter. The medium-metamorphized coal, capable of turning into a plastic state and cake in the range of investigated temperatures (350–450°C), is desulphurized with the greatest difficulty. The chemical mechanism dealing with the transformations of pyretic sulphur present in brown coal differs from similar processes taking place in black coal and anthracite, because FeS2 is converted with the help of hydrogen sulphide formation during desulphurization.

Surface property effects on dropwise condensation heat transfer from flowing air-steam mixtures to promote drainage

In this study, the effect of a partially structured Ti-coated plate surface on droplet drainage and heat transfer in dropwise condensation in a compact plate heat exchanger is investigated. In the presence of high concentrations of inert gases, heat transfer is governed by vapor diffusion and condensate drainage is of major importance. A structured coating of the condenser plates is applied to create two coexisting dropwise condensation patterns. The structured Ti-coating constrains drainage and introduces directed surface energy “gradients”, 1-D binary patterns. The condenser with the partially structured coating is compared with two equally sized condensers: a full PVDF and a fully Ti-coated PVDF condenser. It is found that drop drainage is promoted by oriented Ti-coated tracks with a width of approximately the diameter of the maximum drop size to such a degree that the heat transfer performance is practically the same as that of a fully Ti-coated exchanger. Design recommendations are given.

Condensation of steam-air mixture in a film-type apparatus

The results from a study of heat transfer during condensation of steam and air-steam mixture in a film-type tubular apparatus with heat transfer enhancement by streamlined bodies are presented. Dependencies for calculating the heat transfer coefficient are proposed.

Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.

Experimental investigation of spray induced gas stratification break-up and mixing in two interconnected vessels

To analyze the effect of containment spray on gas mixing and depressurization, two experiments (ST3_1 and ST3_2) were performed with two interconnected vessels. These experiments were conducted in the frame of the OECD/SETH-2 project using the PANDA facility. The vessels were preconditioned such that a helium-rich layer is formed in the upper section of the first vessel, henceforth referred to as Vessel-1. In the case of the first experiment (ST3_1), the remaining volume of Vessel-1 and the entirety of the second vessel, Vessel-2, were filled with pure steam. For ST3_2, the second experiment presented here, pure steam was replaced with a steam–air mixture instead. Water was injected from the top of Vessel-1 with a spray nozzle projecting downwards. Transient behavior of system pressure, as well as global redistribution of gases is investigated. The results reveal that spray activation is very effective in containment system depressurization. Additionally it is found that the depressurization occurs at a higher rate for the systems containing more steam and less non-condensible gas. The depressurization rate gradually slows down, however, as the steam concentration decreases due to condensation, and non-condensible gases spread over the vessel system. It is also observed that the spray activation initiates the breakup of the helium-rich layer. The composition of the gas atmosphere plays a crucial role in determining the initiation time of the breakup; the presence of large amounts of non-condensible gas such as air delays the beginning of the helium layer breakup by approximately 200 s. The downward component of spray momentum causes the entrainment and the recirculation of the ambient gas atmosphere. Together with the entrainment and condensation effect, spray activation influences the gas mixture density in Vessel-1 and this generates a driving force for inter-compartment flow. As a result of this, an increase of helium–rich gas mixture is observed in the regions far away from the spray, i.e., in Vessel-2.

Dropwise condensation from flowing air–steam mixtures: Diffusion resistance assessed by controlled drainage

In this study, the initial phase of drop growth, when diffusion is not limiting, is artificially made more important. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and applied. The dropwise condensation process is frequently interrupted upon which nucleation restarts upon each sweep. Condensate growth and surface temperatures are assessed by simultaneous video and infrared recordings. Cold wakes downstream of big drops on the condenser plate were observed. A single controlled droplet removal action enables a ‘reset’ of the condenser surface. This allowed measurement of droplet growth histories. It is found that droplet growth follows a power law of D ∝ t β , with β increasing with increasing ω vap, in . Direct contact condensation on drops at condenser plate dominates drop growth. The main finding is that the total heat transfer resistance decreases with increasing droplet removal frequency, while two measures for mass transfer simultaneously increase. Increasing diffusion limitation is one explanation for the observed decreasing mass transfer rate with time. After initial fast growth of drops, the slight increase in interfacial temperature observed offers another explanation.

G25 水蒸気一空気混合気の水平円管外面への強制対流凝縮実験(G2熱物質移動・相変化)

G25 水蒸気一空気混合気の水平円管外面への強制対流凝縮実験(G2熱物質移動・相変化)

Air-steam gasification of sewage sludge in a bubbling bed reactor: Effect of alumina as a primary catalyst

Numerous references can be found in scientific literature regarding biomass gasification. However, there are few works related to sludge gasification. A study of sewage sludge gasification process in a bubbling fluidised bed gasifier on a laboratory scale is here reported. The aim was to find the optimum conditions for reducing the production of tars and gain more information on the influx of different operating variables in the products resulting from the gasification of this waste. The variables studied were the equivalence ratio (ER), the steam-biomass ratio (SB) and temperature. Specifically, the ER was varied from 0.2 to 0.4, the SB from 0 to 1 and the temperature from 750°C (1023 K) to 850°C (1123 K). Although it was observed that tar production could be considerably reduced (up to 72%) by optimising the gasification conditions, the effect of using alumina (aluminium oxide, of proven efficacy in destroying the tar produced in biomass gasification) as primary catalyst in air and air-steam mixture tests was also verified. The results show that by adding small quantities of alumina to the bed (10% by weight of fed sludge) considerable reductions in tar production can be obtained (up to 42%) improving, at the same time, the lower heating value (LHV) of the gas and carbon conversion.

Behaviour of dolomite, olivine and alumina as primary catalysts in air–steam gasification of sewage sludge

Sewage sludge gasification assays were performed in an atmospheric fluidised bed reactor using air and air–steam mixtures as the gasifying agents. Dolomite, olivine and alumina are three well known tar removal catalysts used in biomass gasification processing. However, little information is available regarding their performance in sewage sludge gasification. The aim of the current study was to learn about the influence of these three catalysts in the product distribution and tar production during sewage sludge gasification. To this end, a set of assays was performed in which the temperature (750–850 °C), the in-bed catalyst content (0, 10 and 15 wt.%) and the steam–biomass ratio (SB) in the range of 0–1 were varied with a constant equivalence ratio (ER) of 0.3. The results were compared to the results from gasification without a catalyst. We show that dolomite has the highest activity in tar elimination, followed by alumina and olivine. In addition to improving tar removal, the presence of water vapour and the catalysts increased the content of H 2 in the gases by nearly 60%.

High Accuracy Acoustic Relative Humidity Measurement inDuct Flow with Air

An acoustic relative humidity sensor for air-steam mixtures in duct flow is designed and tested. Theory, construction, calibration, considerations on dynamic response and results are presented. The measurement device is capable of measuring line averaged values of gas velocity, temperature and relative humidity (RH) instantaneously, by applying two ultrasonic transducers and an array of four temperature sensors. Measurement ranges are: gas velocity of 0–12 m/s with an error of ±0.13 m/s, temperature 0–100 °C with an error of ±0.07 °C and relative humidity 0–100% with accuracy better than 2 % RH above 50 °C. Main advantage over conventional humidity sensors is the high sensitivity at high RH at temperatures exceeding 50 °C, with accuracy increasing with increasing temperature. The sensors are non-intrusive and resist highly humid environments.

The panda tests 9 and 9bis investigating gas mixing and stratification triggered by low momentum plumes

This paper presents the results of two tests, performed in the PANDA facility in the frame of the OECD SETH project. The Test 9 and Test 9bis were characterized by the same initial Froude number ( Fro ∼ 1) and were performed at constant pressure in the vessels. The vessels were initially filled with air. During the tests the plumes were generated by injecting a constant amount of steam in Vessel 1 and continuous venting of a steam–air mixture from Vessel 2. In Test 9bis the temperature of the injected steam and the initial containment wall and fluid temperatures where chosen to reach later the conditions for (wall) condensation. The on-set of condensation occurred in Test 9bis at the top of Vessel 1 when, due to the steam injection the steam partial pressure increased enough to eliminate the steam superheating. The condensed droplets propagated toward the bottom of Vessel 1, where due to the lower steam partial pressure, a portion of condensate vaporized again. The evaporation had the effect of increasing the steam partial pressure and progressively new conditions for steam condensation were obtained also at the bottom of Vessel 1. The condensation in Test 9bis had the effect of mitigating the reduction of plume buoyancy (which is a function of the air-steam concentration in Vessel 1) and for this reason the overall plume trajectory, steam–air mixing, stratification and transport between the two compartments were affected.

Numerical analysis of hydrogen risk mitigation measures for support of ITER licensing

In the ITER wet bypass scenario, water leakage, air ingress and hot dust (Be, W, and C) in the vacuum vessel could generate combustible hydrogen–air–steam mixture. Hydrogen combustion may threaten the integrity of the ITER VV and lead to radioactivity release. To prevent hydrogen energetic combustion, nitrogen injection system in VV and hydrogen recombination system in the pressure suppression tank (ST) were proposed. The main objectives of this analysis are to study the distribution of hydrogen–air–steam mixtures in the ITER sub-volumes, to investigate the feasibility of the nitrogen injection system to fully inert the atmosphere in the VV and to evaluate the capability and efficiency of the hydrogen recombination system to remove hydrogen in the ST. 3D computational fluid dynamics (CFD) code GASFLOW was used to calculate the evolution of the mixtures and to evaluate the hydrogen combustion risks in the ITER sub-volumes. The results indicate that the proposed hydrogen risk mitigation systems will generally prevent the risks of hydrogen detonation and fast deflagration. However, the atmosphere in ITER sub-volumes cannot be completely inerted at the early stage of the scenario. Slow deflagrations could still generate quasi-static pressures above 1 bar in the VV. The structural impact of the thermal and pressure loads generated by hydrogen combustions will be investigated in future studies.

Exergy Efficiency of Two-Phase Flow in a Shell and Tube Condenser

This study deals with a comprehensive efficiency investigation of a TEMA “E” shell and tube condenser through exergy efficiency as a potential parameter for performance assessment. Exergy analysis of condensation of pure vapor in a mixture of non-condensing gas in a TEMA “E” shell and tube condenser is presented. This analysis is used to evaluate both local exergy efficiency of the system (along the condensation path) and for the entire condenser, i.e., overall exergy efficiency. The numerical results for an industrial condenser with a steam–air mixture and cooling water as working fluids indicate significant effects of temperature differences between the cooling water and the environment on exergy efficiency. Typical predicted cooling water and condensation temperature profiles are illustrated and compared with the corresponding local exergy efficiency profiles, which reveal a direct (inverse) influence of the coolant (condensation) temperature on the exergy efficiency. Further results provide verification of the newly developed exergy efficiency correlation with a set of experimental data.

Jet-plume condensation of steam–air mixtures in subcooled water, Part 1: Experiments

A scaled-down, reduced pressure suppression pool was designed to study condensation and mixing phenomena for a LOCA (loss of coolant accident) event in a SBWR (simplified boiling water reactor) design. The scaled-down test facility represented an idealized trapezoidal cross-section, 1/10 sector of the SP (suppression pool) with scaled height ratio of 1/4.5 and volume ratio of 1/400. The facility was instrumented with thermocouples for pool temperature measurements and a high-speed camera for flow visualization. Thermal stratification data were obtained for different pool initial subcooling and steam–air mixture flow rates. A dimensionless boundary map was derived from several experimental runs of pure steam injection to determine conditions when the pool transitions from being a homogeneously mixed volume to being a thermally stratified one. Steam air mixture injection cases for single horizontal venting indicated that above a pool temperature of 40 °C with air mass fraction below 0.5% the pool can attain thermal stratification.

Jet-plume condensation of steam–air mixtures in subcooled water. Part 2: Code model

The system-analysis thermal-hydraulics code, TRACE, was used to model jet-plume condensation of steam and steam–air mixtures in a subcooled pool. The code model was used to compare the pool temperature development in a scaled-down test facility suppression pool (SP) represented by an idealized trapezoidal cross-section and 1/10 sector of a with scaled height ratio of 1/4.5 and volume ratio of 1/400. Thermal stratification and pool mixing data obtained in the experimental test section at different pool initial subcooling and steam/air mixture flow rates were used a validation suite to benchmark against the performance of the code. Agreement of the code simulations to experiments were seen in cases of pool mixing, however, instances of pool thermal stratification and transitional regimes between stratification and mixing were not predicted correctly by the code. The experimental database described in part 1 to this paper provides the benchmark suite to be used for future developmental improvement in areas of the code which may have future application to analysis of thermal heat removal rates in large pools.

Humidification tower for humid air gas turbine cycles: Experimental analysis

Abstract In the HAT (humid air turbine) cycle, the humidification of compressed air can be provided by a pressurised saturator (i.e. humidification tower or saturation tower), this solution being known to offer several attractive features. This work is focused on an experimental study of a pressurised humidification tower, with structured packing. After a description of the test rig employed to carry out the measuring campaign, the results relating to the thermodynamic process are presented and discussed. The experimental campaign was carried out over 162 working points, covering a relatively wide range of possible operating conditions. It is shown that the saturator behaviour, in terms of air outlet humidity and temperature, is primarily driven by, in decreasing order of relevance, the inlet water temperature, the inlet water over inlet dry air mass flow ratio and the inlet air temperature. The exit relative humidity is consistently over 100%, which may be explained partially by measurement accuracy and droplet entrainment, and partially by the non-ideal behaviour of air–steam mixtures close to saturation. Experimental results have been successfully correlated using a set of new non-dimensional groups: such a correlation is able to capture the air outlet temperature with a standard deviation σ = 2.8 K.

Adiabatic fixed-bed gasification of coal, dairy biomass, and feedlot biomass using an air–steam mixture as an oxidizing agent

Concentrated animal feeding operations, such as cattle feedlots and dairies, produce a large amount of manure, cattle biomass (CB), which can be included as renewable feedstock for locally based gasification for syngas (CO and H 2) production and subsequent use in power generation. Experimental results on effects of bed temperature and gas composition on the higher heating value (HHV) and energy recovery are presented for dairy biomass (DB) gasification using air and air–steam as oxidizers. Some experimental data are compared with adiabatic gasification modeling which includes atom balance conservation for assumed product species and chemical equilibrium analysis. Wyoming sub-bituminous coal (WYC) and Texas Lignite coal (TXL) are used as standard fuels for comparison purposes in modeling studies. Two main parameters are investigated in this study. One is the modified equivalence ratio (ER M) defined as the ratio of stochiometric oxygen to total oxygen supplied in the oxidizing mixture of air and steam. The second is a measure of how much steam is in the oxidizer and is called the air steam ratio (ASTR), which is defined as the ratio of oxygen supplied in the air to the total oxygen supplied in the oxidizer. The results suggested that gasification of CB and coals under higher ER M yield elevated concentrations of CO and CH 4, and low percentages of H 2 and CO 2, while higher ASTRs (less steam) produced mixtures poor in H 2, CO 2, and CH 4 and rich in CO with lower HHV. It was also found that FB and DB produced higher amounts of H 2 than WYC and TXL under the same ER M and ASTR.